Insulating and superconducting states of monolayer WTe₂

In monolayer WTe₂, low symmetry, band inversion, heavy hole mass, electron-hole compensation, and strong spin-orbit coupling together provide a promising setting for nontrivial correlated and topological phenonema. I will summarize the experimental evidence for some of these phenomena. Undoped, this material behaves as a two-dimensional topological insulator, while at a surprisingly low level of electron doping it becomes superconducting below 1 K. Anisotropic magnetoresistance measurements of the edge conduction in the insulating state demonstrate that the edges states are indeed helical, and also reveal a tilted-Ising-like electron spin polarization in common with the bulk bands from which the superconducting state must be built. Some measurements suggest that the helical edge states of the topological insulator persist in the metallic and even in the superconducting regime, while being very weakly coupled to it. In addition, many pieces of evidence taken together indicate that the insulating behavior is driven by electron-hole pairing in what would otherwise be a semimetal, with the possible formation of zero-momentum excitonic insulator ordering near the neutral state that competes with the superconductivity at moderate doping.